Data transmission method serial bus system and switch-on unit for a passive station

ABSTRACT

In a serial bus system data in the form of telegrams, representing process images of control tasks of the active station, are transmitted to the connected passive stations, and the process data are allocated to the process images in the passive station.

This application claims priority to International Application No.PCT/EP02/14423 filed on Dec. 17, 2002, which claims priority to Germanpatent application DE 101 63 342.4 filed on Dec. 21, 2001.

FIELD OF THE INVENTION

The invention relates to a method for transmitting data on a serial busbetween at least one active station and at least one passive station, toa correspondingly configured serial bus system, and to a switch-on unitfor a passive station in such a serial bus system.

BACKGROUND OF THE INVENTION

Serial bus systems are being used more and more for production andprocess automation, wherein the decentralized devices of peripheralmachines, such as I/O modules, measurement transducers, drives, valves,and operator terminals, communicate by means of a powerful real-timecommunications system with automation, engineering, or visualizationsystems. In this way, all instruments are linked to each other by meansof one serial bus, preferably a field bus, wherein the data exchange bymeans of the bus is usually performed on the basis of the master-slaveprinciple.

Here, the active stations on the bus system are the control devices.They are in possession of a bus-access token and determine the datatransfer on the bus. The active stations are also called master devicesin the serial bus system.

In contrast, passive stations are usually the peripheral machines. Theyreceive no bus-access token, i.e., they are allowed only to acknowledgereceived messages or transmit messages to active stations upon a requestby this active station. Therefore, passive stations are also calledslave devices in the serial bus system.

In general, the master devices contain a field-bus switch-on device,which represents the link between the central data processing in themaster device and the field-bus network, and which performs busmanagement. Thus, the switch-on device that is often configured as aseparate assembly also implements the complete bus protocol. The slavedevices in turn feature an interface module, which converts the datafrom the slave device into the data format of the field-bus system.Therefore, these interface modules also require only a small portion ofthe bus protocol.

In automation technology, field-bus systems are used according to themaster-slave principle with a wide variety of different transmissionrules. For cyclical field-bus systems, e.g., Profibus-DP, ControlNet,FIP-IO, or Interbus-S, the data is transmitted in a cycle independent ofwhether the data has been changed. In contrast, in acyclical field-bussystems, like those known for CAN systems, data is only transmitted ifthe data has been changed or if the data transmission is explicitlytriggered by the master device.

Further distinctions are also made between stationary-oriented field-bussystems, like Profibus-DP, for which a master device sends a message toa slave device that then acknowledges or answers this message, andmessage-oriented field-bus systems, like the CAN system, which isdistinguished in that the master device outputs unacknowledged messagesonto the bus, which can then be processed by all stations. Furthermore,there are also bus-oriented field-bus systems, like Interbus-S, forwhich the master device transmits a message with all of the data for allof the attached slave devices, wherein the site of the data for therespective slave device is determined by its position In the messageblock.

Control processes, especially for automation systems used in production,are usually assembled from one or more tasks, which are in generalperformed in a cycle. Such tasks are performed in a field-bus system sothat the slave devices, which represent the peripheral machines, deliverinput process data by means of the field bus to the master device, whichfunctions as a process controller. In the master device, the outputprocess data is then generated corresponding to the task to beperformed, and is transmitted by the field-bus system to the slavedevices.

In the conventional transfer of process data, the process data of theslave devices is output according to the bus protocol used by theinterface unit of the appropriate slave device onto the field bus, andreceived by the field-bus switch-on device as the interface of themaster device with the field bus. The field-bus switch-on device in themaster device then builds a process image for the appropriate task fromthe received process data of the slave devices, and forwards thisprocess image to the data processing unit in the master device. The dataprocessing unit in the master device in turn creates an output processimage, which is output to the field-bus switch-on device, on the basisof the input process image according to the task to be performed. Thefield-bus switch-on device then determines the process data for theindividual slave devices from this process image and transmits thisprocess data according to the provided bus protocol to the appropriateslave device by means of the field bus.

For the known field-bus systems, the process data transfer between themaster device and the slave devices is performed conventionally, so thatthe process data for the individual process devices is assembled intoseparate data packets, which are also called telegrams in the following,and the slave devices are then addressed individually.

For the process data transfer, the field-bus switch-on device in themaster device must therefore perform a complex allocation between theprocess data transmitted from and to the slave devices and the input andoutput process images to be allocated to each task. However, thisallocation, which is also called mapping in the following, of processdata of the slave devices to the process images, which are used in themaster device and which are formed in the field-bus switch-on device ofthe master device, requires a high processing effort. Therefore, acomplex mapping algorithm and protocol sequence must also be installedin the field-bus switch-on device. This applies even more to openfield-bus systems, which are designed so that stations can be removedfrom the overall system and inserted into the system without greatexpense.

The required conversion of the process data of the individual slavedevices into process images in the field-bus controller also leads tosignificantly delayed processing of processes, especially when severaltasks are supposed to be performed simultaneously.

This applies above all when a control application consists of severalpartial processes, each of which is allocated to a different slavedevice. For conventional field busses, the process image allocated tothe control application is then divided into corresponding individualtelegrams for the appropriate slave devices. In contrast, in the eventthat the process data for one task has to be made available to severalslave devices, because all of these slave devices are supposed toperform this task, the field-bus controller must then build a uniquetelegram with the process data for each slave device.

Furthermore, if the individual partial processes have different cycletimes, conventional data transmission methods on the field bus cannotadapt the bus load to the individual partial processes according to thedifferent cycle times, and thus cannot achieve an optimal use of thebus.

In addition, conventional field-bus systems can only with muchdifficulty provide flexible adaptation of the telegrams to the processdata lengths required in the slave devices. Thus, if a slave devicerequires only 1-bit process data, in general an additional bus couplerwith a local bus network is necessary in order to convert this 1-bitprocess data into the telegrams of the standard field bus.

The problem of the present invention is to refine the known serial bussystems, especially field-bus systems, so that flexible datatransmission is possible with reduced data processing effort.

This problem is solved by a method according to claim 1, a serial bussystem according to claim 7, and a switch-on unit according to claim 11.Preferred embodiments are given in the dependent claims.

SUMMARY OF THE INVENTION

According to the invention, for transmitting data to a serial bus systemwith at least one active station and at least one passive station, datain the form of telegrams is transmitted between the active and thepassive stations, wherein the telegrams each contain a logical addressin order to characterize the transmitted data, wherein the passivestation contains an allocation table in which logical addresses oftelegrams are correlated with physical addresses of data storage regionsof the passive station, and wherein during passage of a telegram on theserial bus the station compares the logical address of the circulatingtelegram to the logical addresses stored in the allocation table andperforms a data exchange with the telegram if a match is identified.

According to the invention, the design of the serial bus or the datatransmission onto this serial bus, for which each station can use itsswitch-on unit to remove or insert any pre-configured data into a dataflow transmitted in the form of telegrams, provides high flexibility forthe data transmission between the stations with simultaneously low dataprocessing effort. In particular, a simplified process data transmissionof control applications is possible between decentralized devicesattached to the serial bus.

For the design according to the invention, each decentralized passivedevice attached to the serial bus uses the allocation table to determineindividually which data in the telegrams transmitted on the bus isallocated to the respective decentralized device. In addition, with thehelp of the telegrams and the allocation tables provided in the passivestations, data can be transmitted according to the respective processrequirements and independently of its physical position.

According to the invention, it is especially possible, with the help ofthe telegrams, to adapt the data flow to the respective requirements ofthe passive stations, wherein, e.g., telegrams with 1-bit process datacan also be realized for passive stations. Thus, economical serial bussystems can be realized without additional bus couplers and local busnetworks.

Furthermore, with the help of the data transmission according to theinvention, the data flow on the serial bus is adapted to the appropriatecycle times of partial processes, wherein an optimal use of the bus canbe achieved.

Furthermore, the configuration according to the invention avoids acomplicated mapping process in the active station, with which theprocess data allocated to the individual passive stations is correlatedwith the corresponding process images of the control application. Themapping process is already performed in the passive stations themselves,namely by means of the assignment table, wherein the data processingeffort in the active station is significantly reduced, and thusconsiderable time savings are achieved when performing the controltasks. In addition, the device and implementation effort issignificantly reduced by shifting the assignment process into thepassive stations.

According to a preferred embodiment, in the interface unit of thepassive station the stored assignment table is divided into registerdata sets, each of which gives a logical address of data transmitted bythe serial bus in a telegram, a physical address of the data memoryassigned to the data in the passive station, and a data transmissionmode. The configuration according to the invention of the mappingprocess and of the data transmission in the passive station provides anespecially simple and fast-acting assignment process for only a smallimplementation and protocol effort in the passive station. Inparticular, with this design of the assignment process, it is possiblein the passive station to perform the data transfer between thetelegrams circulating on the serial bus and the data memories assignedto the corresponding passive stations in a simple way.

According to another preferred embodiment, the assignment table in thepassive station is generated during an initialization phase, wherein theactive station reports to the passive station the logical addresses ofdata relevant for the passive station in the individual telegrams, andthe passive station then correlates the logical addresses with thecorresponding physical address of data memories. This configurationenables high flexibility for the data transmission, and in particularadaptation to the respective desired control application. It can beindividually reported to the passive stations which process data istransmitted from the active station, i.e., by the control device, andwhere this process data is located in the telegrams circulating on theserial bus.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention is explained in more detail with reference to the attacheddrawings. Shown are:

FIG. 1, a schematic diagram of a serial bus system;

FIG. 2, a conventional process data transfer in a serial bus system,which is used for control applications;

FIG. 3, a schematic diagram of a serial bus system with interface unitsaccording to the invention in the slave devices;

FIG. 4, a process data transfer according to the invention in a serialbus system, which is used for control applications;

FIG. 5, an arrangement of telegrams with the process images of tasks ona cyclical field bus in the serial bus system;

FIG. 6, a flow chart in an interface unit of a slave device according tothe invention when a read telegram is received; and

FIG. 7, a flow chart in an interface unit of a slave device according tothe invention when a write telegram is received.

DETAILED DESCRIPTION OF THE INVENTION

More and more field-bus systems are being used in automation technology,wherein distributed devices of the peripheral machines communicate withautomation, engineering, and visualization systems by means of a fieldbus. One such field-bus system is shown in FIG. 1.

The field-bus system has a serial bus 1, which can be, e.g., anelectrical line, an optical fiber, or also a radio cable. All of thestations are attached to this bus 1, wherein a distinction can be madebetween active and passive stations. The active stations on thefield-bus system are the master devices 2, which determine the datatraffic on the bus. One such master device is, e.g., an industrial PC,which operates as a process control computer in a manufacturing process.This master device 2 has a bus access token and can output data onto thebus 1 without an external request. The passive stations on the bussystem are the peripheral machines, e.g., I/O devices, valves, drives,and measurement transducers. They act as slave devices 3 and receive nobus-access token, i.e., they are allowed only to acknowledge receivedmessages or transmit messages to a master device upon a request by themaster device.

The master device 2 and the slave devices 3 can be attached directly tothe bus 1 or can be connected to this bus by means of standaloneinterface components. The type of data transmission on the bus 1 isdetermined by a communications protocol, with a wide variety of protocolarchitectures being possible as a function of the transmission physicsselected for the serial bus 1. This bus protocol is implemented in themaster device 2, preferably in a field-bus switch-on device 23. The busprotocol parts necessary for the slave devices 3 are stored in anallocation unit 33.

Processes in automation technology usually consist of various partialprocesses, which are influenced by different time behaviors. Thus thecontrol applications, which control, monitor, and regulate theseprocesses, generally have several cyclical control tasks with differentcycle times which each provide input and output process images. Here theprocess data is usually distributed to many passive stations, which formthe interfaces to the process. The process signals are here updated viathe serial bus, depending on the field-bus system, with differentmethods in the master device with the control applications and in thepassive stations, and are represented in process images.

FIG. 2 shows as an example a conventional data transfer between themaster device 2, which contains the control application and thefield-bus controller, and the slave devices 3 for performing threecontrol tasks, which are designated as task 1, task 2, and task 3 in themaster device. To perform these control tasks, input and output processimages are used in the master device 2, wherein in FIG. 2 the inputprocess image for task 1 is represented as process data 1, the inputprocess image for task 2 is represented as process data 2, and the inputprocess image for task 3 is represented as process data 3. These inputprocess images are generated by the field-bus switch-on device 23 in themaster device 2 on the basis of the process data supplied by theallocation units 33 of the individual slave devices 3 via the field bus.

In the prior art, in order to allocate the data relevant for the slavedevices 3 to the individual tasks, thus, the process data to the processimages, an allocation method called mapping of the process data of theindividual slave devices to the process images is performed. However,this conventional mapping algorithm, which is performed by the field-busswitch-on device 23 in the master device 2, is complex, especially foropen field-bus systems in which slave devices can be switched on and offarbitrarily. Furthermore, the process images for the individual tasksare generated by the field-bus switch-on device 23 one after the other,which can lead to considerable time losses in execution of the tasks.

For the conventional data transfer shown in FIG. 2, according to theillustrated control application, process data of task 1 must beexchanged with the slave devices 1, 2, and 5, process data of task 2must be exchanged with the slave devices 2, 4, and 6, and process dataof task 3 must be exchanged with the slave devices 1, 3, and 4, theindividual slave devices each being assigned only parts of the processdata allocated to the individual tasks because the respective slavedevices only execute partial processes of the tasks. In conventionalfield-bus systems, however, the telegrams cannot be adapted so that therespective slave devices receive only the process data allocated tothem. In particular, the telegrams cannot be formed so that the dataflow on the field bus is adapted to the different cycle times of thepartial processes.

In order to be able to optimally adapt the data transfer to the slavedevices for the respective control applications, in this way keeping toa minimum the delay times for mapping the process data of the slavedevice in the process images of the master device, and in order toachieve a simple implementation of the transmission protocol in themaster-slave bus system, according to the invention a data flow in theform of telegrams is made available on the serial bus, so that eachslave device can remove or insert exactly the data destined for it.

According to the invention, the mapping is then performed directly inthe corresponding slave devices, a parallel mapping process beingpossible in all the slave devices, which reduces the load on the masterdevice and provides a considerable time advantage in execution of thecontrol tasks. In addition, the device and implementation effort in thefield-bus system is considerably reduced by the mapping processaccording to the invention.

FIG. 3 shows a possible configuration of the invention, for which acyclical field bus, e.g., a Profibus-DP, is used as a serial bus. Forone such cyclical field bus, the process data is transmitted on the busline 1 with the cycle time allocated to the corresponding control task,wherein the data transmission is performed independent of whether thedata has changed within this cycle time. However, the invention is notlimited to such a cyclical field bus. Alternatively, the field bus canalso be configured acyclically, e.g., a CAN bus, for which the processdata is transmitted only if it has changed or if the data transmissionis explicitly triggered by the master device.

As FIG. 3 shows, the master device 2 and each of the slave devices 3have a transmitter 22, 32 for output of data onto the bus line 1, and areceiver 21, 31 in FIG. 3, for receiving data from this bus line 1.Transmitters and receivers for a device can be built in a standaloneunit in the form of a bus switch-on device, or also can be integratedinto the master device or the slave devices themselves. The bus 1 can beconfigured as an electrical line or also as optical fibers.

The transmitter 21 and the receiver 22 of the master device 2 areconnected to the field-bus switch-on device 23 in the master device 2,which implements the overall bus protocol and which controls datatransfer via the field bus. This field-bus switch-on device 23 isconnected by means of an interface to a data-processing unit in themaster device 2, in order to transfer to the master device 2 the inputprocess images necessary for processing the control tasks, or to receivethe output process images generated by the master device.

The slave devices 3 each contain the allocation unit 33 which isconnected to the receiver 31 and the transmitter 32. In this allocationunit 33, which is also called FMMU (Field-bus Memory Management Unit) inthe following, the part of the bus protocol necessary for the respectiveslave device is implemented. The allocation unit 33 in the slave devices3 controls the data transfer between the data packets, which are locatedon the bus line 1 and which are also called telegrams in the following,and the process data stored in a data memory 34 of the slave device 3,which is also called physical memory in FIG. 3. This process data canbe, e.g., measurement data or control data for performing amanufacturing process by the corresponding slave device.

The FMMU unit 33 in the slave device 3 can be formed as a standalonestructural unit or can be integrated into a control unit. Furthermore,there is the ability to define the functions of the FMMU unit 33, e.g.,in the form of an ASIC chip for the respective slave device or also toimplement a software solution in a microcontroller in the slave device.

FIG. 4 shows schematically a data transmission that can be performed onthe field-bus system shown in FIG. 3. As in the conventional processdata transfer shown in FIG. 2, it is assumed that three control tasks,task 1, task 2, and task 3 are performed by the master device 2. Aprocess image with process data 1, 2, 3 is allocated to each of thesecontrol tasks. These process images for the individual tasks areexchanged between the data processing unit of the master device 2 andthe field-bus switch-on device 23, wherein the process images are thenoutput with the help of the transmitter 21 by the field-bus switch-ondevice 23 onto the field bus 1, or are received by this switch-on deviceby means of the receiver 22.

For the cyclical field bus shown in FIG. 3, the process images for themaster device 2 are transmitted in a process-data-oriented way, i.e.,the process image of one task is divided according to size into one ormore telegrams that circulate on the field bus. The process dataallocated to the respective slave devices of the process imagestransmitted as telegrams on the field bus are read under the control ofthe FMMU unit 33 in the corresponding slave device 3 by the receiver 31of the slave device 3 or output by means of the transmitter 32 of theslave device 3. Allocation of the process data of the slave device 3 tothe process images, which are transmitted in telegram form on the fieldbus, is here performed by the FMMU unit 33 in the slave device 3, sothat this allocation of the process data to the process images can beperformed by all slave devices on the field bus simultaneously while thetelegram is circulating.

For the data transfer shown in FIG. 4, it is possible in a simple way toconfigure the slave devices according to the control application so thattheir process data can be transmitted with various telegrams. In aninitialization phase, the master device reports to the slave devicewhich telegrams carry its process data, in which part of the allocateddata memory the process data is stored, and where this process data islocated in the telegrams. During the cyclical control operation,telegrams with process images are then transmitted via the field bus,from which the slave devices remove their output process data or inserttheir input process data.

As shown in FIG. 4, a data transfer of the process data of task 1 to theslave devices 1, 2, and 5, the process data for the task 2 to the slavedevices 2, 4, and 6, and the process data of task 3 to the slave devices1, 3, 4, and 6 can be performed simply and economically with the aid ofthe telegrams. Furthermore, according to the invention, the datatransfer can be configured so that the slave devices 1 and 2 provideonly a part of the process data with telegram 1, the other parts of theprocess data then being transmitted with telegrams 2 and 3. The datatransmission process shown in FIG. 4 is particularly suitable forcontrolling an application that consists of three partial processes,wherein, for the first partial process the signals change, e.g., every100 msec, for the second partial process the signals change every 10msec, and for the third partial process the signals change every 1 msec;and the first partial process requires process data from slave devices1, 2, and 5, the second partial process requires process data from slavedevices 2, 4, and 6, and the third partial process requires process datafrom slave devices 1, 3, 4, and 6.

Through the data transmission according to the invention, there is theability to implement telegrams for slave devices that require only 1-bitprocess data. Therefore, it is no longer necessary, as in conventionalfield-bus systems, to convert the telegrams from a standard field businto a local bus to the slave devices that transmits 1-bit process data,so that a simplified and economical slave switch-on device is realized.

Through the flexible configuration of the telegrams, it is also possiblefor the data flow on the field-bus system to be adapted to partialprocesses with different cycle times. Thus, by means of appropriatelydesigned telegrams without anything additional, data transmission can beperformed for which process data for task 1 must be transmitted everymsec, wherein the transmission period of the process data equals 0.5msec, the process data of task 2 is transmitted every 10 msec, whereinthe transmission period of the process data equals 2 msec, and theprocess data of task 3 is transmitted every 100 msec, wherein thetransmission period equals 10 msec. By adapting the telegramsappropriately, a data transmission with an 80% bus load is achieved,wherein the process data for tasks 2 and 3 are each transmitted inseveral telegrams because for tasks 2 and 3, only 0.5 msec is availablein each period of 1 msec.

Through this allocation of process data to the process imagescirculating on the field bus, this allocation, being shown schematicallyin FIG. 4, performed by the respective interface units in parallel inall of the slave devices, the load on the field-bus switch-on device inthe field-bus system is reduced significantly. In addition, with thehelp of a simple mapping algorithm in the FMMU unit 33, the allocationprocess between the process data of the respective slave device 3 andthe associated process images of the master device 2 can be performedwith only a low protocol and implementation effort.

FIG. 5 shows in detail the arrangement of telegrams on the cyclicalfield bus used in the embodiment of FIG. 3. Here, the field bus can beconsidered as a kind of logical memory, wherein the process images ofthe individual tasks supplied as telegrams can be divided arbitrarily inthe logical memory. FIG. 5 shows schematically one possible arrangementof telegrams in the logical memory of the field bus for the three tasksof the master device shown in FIG. 4. Here, free regions in the logicalmemory of the field bus can be provided between the individual tasks,independent of the process images.

Each telegram of a task is built from two parts, a head part, which actsas a logical address block, and a data part. The logical address blockof the telegram is in turn divided into three memory regions, whereinthe first memory section gives the data transmission type, i.e., whetherthe data should be written by the master device into the slave device(write command) and/or whether the data should be transmitted from theslave device to the master device (read command). In a second memorysection, the data contained in the telegram is characterized with anaddress. As an additional memory region, the logical address blockcontains information on the length of the data contained in thetelegram. A data part in which the process data is contained adjoins theaddress block.

In order for the FMMU unit 33 of the corresponding slave device 3 torecognize the process data allocated to it in the telegrams on theserial bus, this FMMU unit 33 contains an allocation table with severalregister data sets that correlate the logical memory of the field buscontaining the process images to the physical memory in thecorresponding slave device containing the process data. Each registerdata set comprises the following registers, which describe an assembledmemory region on the field bus considered as a logical memory: startingaddress of the logical memory (field bus); ending address of the logicalmemory (field bus); starting address of the physical memory (slave);type of data transmission “write,” “read,” or “read/write.”

Each register data set thus indicates the beginning of the dataallocated to the corresponding slave device in the logical memory of thefield bus. By fixing the data end in the register set, the length of thedata allocated to the corresponding slave device is then set in thelogical field bus memory. Instead of an end address, the register dataset could also contain information on the length of the data region inthe logical memory. The information contained in the register data setconcerning the beginning of the data set in the physical memory of theslave device then provides the allocation of the data contained in thetelegrams on the field bus to the data in the slave device. The type ofdata transmission in the register set then determines whether a writingor reading process or a combined reading-writing process is to beperformed between the slave device and the field bus.

The register data sets in the slave devices are generated in aninitialization phase of the field-bus system. During the cyclical datatraffic on the field bus, the currently transmitted region of the fieldbus considered as logical memory is then compared with the register datasets of the interface unit in the corresponding slave device by acomparison unit. If there is a match, the corresponding process data isthen read from the telegram on the field bus or input into this telegramwith the help of an access unit.

FIG. 6 shows the processing sequence in the interface unit of a slavedevice when a read telegram is received. The head parts of the telegramscirculating on the field bus are compared in the interface unit with thestored register data sets. If this comparison shows that the data partof the telegram contains regions that are allocated to the slave device,these data regions are read from the physical memory of the slave deviceand input into the corresponding telegram on the field bus.

FIG. 7 shows the processing sequence in the FMMU unit of a slave devicewhen a write telegram is received. Here, again the head parts of thetelegrams on the field bus are compared with the register data sets. Ifa match is determined and the register data set indicates that a writingprocess is to be performed, the corresponding data from the telegram onthe field bus is then read and stored in the physical memory of theslave device.

As an alternative to the telegram configurations shown in FIGS. 6 and 7,there is also the possibility to process a read/write telegram in theslave device. After comparison of the head part of the telegram on thefield bus with the register data sets, if a match is determined, thenthe corresponding data can be read from the telegram by the slave devicewhile the telegram is circulating and written into the physical memoryof the slave device, and simultaneously data regions from the physicalmemory of the slave device can be read and written to the correspondinglocations of the telegram. Here, the processing sequence is preferablylimited so that the maximum amount of data that can be written into thetelegram corresponds to the amount of data that can be read from thetelegram.

Alternatively, it is also possible to use, instead of a telegram with anaddress region, a simple logical address in the form of a logical numberfor addressing the respective slave device, this then being storedtogether with an offset and a data length in the allocation table of theFMMU interface.

The mapping process performed according to the invention by theinterface units of the slave devices can be used for all known types ofserial bus systems. By performing the mapping process between theprocess images and the process data directly in the slave device, thedelay times for performing control tasks can be kept as low as possiblebecause the mapping can be realized in all slave devices in parallel,and thus the load on the system as a whole can be reduced significantly.

1. Method for transmitting data on a serial bus between at least oneactive station and at least one passive station, the data beingtransmitted in the form of circulating telegrams between the activestation and the passive station, the method comprising the steps of:providing one or more circulating telegrams flowing in a circular pathformed by the serial bus, with each circulating telegram starting at anactive station, sequentially moving from the active station to thepassive station on the serial bus and, after sequentially passingthrough the passive station on the serial bus, returning to the activestation from which it originated, each of the circulating telegramsbeing outputted onto the serial bus by the active station and containinga logical address and a data portion; providing an assignment table inthe passive station, the assignment table containing logical addressesof the circulating telegrams, the logical addresses being correlatedwith physical addresses of data-storage areas of the passive station;and while the circulating telegram is passing through the passivestations, comparing the logical address of each circulating telegramwith the logical addresses stored in the assignment table of the passivestation, and if there is a match, performing a data exchange between thedata portion of the circulating telegram and the data-storage area ofthe passive station, which has the physical address assigned to thelogical address.
 2. Method according to claim 1, wherein the assignmenttable has register data sets, each set giving a logical address of datatransmitted in a telegram circulating on the serial bus, a physicaladdress of the data storage area assigned to the data in the passivestation, and a data transmission type.
 3. Method according to claim 2,wherein each register data set contains a starting address of data inthe data portion of the telegram circulating on the serial bus, anending address of the data in the data portion or a data length in thedata portion of the telegram circulation on the serial bus, a startingaddress of the assigned data storage area in the passive station, and alabel characterizing the data transmission type as a read and/or writeprocess.
 4. Method according to claim 3, wherein the assignment table inthe passive station is during an initialization phase.
 5. Methodaccording to claim 4, wherein in the initialization phase the activestation reports to the passive station the logical addresses of datarelevant for the passive stations in the circulating telegrams and thepassive station assigns to the logical addresses respective physicaladdresses of corresponding data storage areas in the passive station. 6.Method according to claim 5, wherein the serial bus is a field bus, datain the active station is provided in the form of process images forcontrol tasks, data in the passive station is provided in the form ofprocess data, telegrams with the process images are transmitted on thefield bus, and for a data exchange between the field bus and the passivestation, the process data in the passive station is assigned to thecorresponding process images in the telegrams circulating on the fieldbus.
 7. Method according to claim 3, wherein the serial bus is a fieldbus, data in the active station as provided in the form of processimages for control tasks, data in the passive station is provided in theform of process data, telegrams with the process images are transmittedon the field bus, and for data transfer between the field bus and thepassive station, the process data in the passive station is assigned tothe corresponding process images in the telegrams circulating on thefield bus.
 8. Method according to claim 4, wherein the serial bus is afield bus, data in the active station is provided in the form of processimages for control tasks, data in the passive station is provided in theform of process data, telegrams with the process images are transmittedon the field bus, and for data transfer between the field bus and thepassive station, the process data in the passive station is assigned tothe corresponding process images in the telegrams circulating on thefield bus.
 9. Method according to claim 2, wherein the assignment tablein the passive station is generated during an initialization phase. 10.Method according to claim 9, wherein in the initialization phase theactive station reports to the passive station the logical addresses ofdata relevant for the passive stations in the circulating telegrams andthe passive station assigns to the logical addresses respective physicaladdresses of corresponding data storage areas in the passive station.11. Method according to claim 10, wherein the serial bus is a field bus,data in the active station is provided in the form of process images forcontrol tasks, and data in the passive station is provided in the formof process data, telegrams with the process images are transmitted onthe field bus, and for data transfer between the field bus and thepassive station, the process data in the passive station is assigned tothe corresponding process images in the telegrams circulating on thefield bus.
 12. Method according to claim 9, wherein the serial bus is afield bus, data in the active station is provided in the form of processimages for control tasks, and data in the passive station is provided inthe form of process data, telegrams with the process images aretransmitted on the field bus, and data transfer between the field busand the passive station, the process data in the passive station isassigned to the corresponding process images in the telegramscirculating on the field bus.
 13. Method according to claim 2, whereinthe serial bus is a field bus, data in the active station is provided inthe form of process images for control tasks, data in the passivestation is provided in the form of process data, telegrams with theprocess images are transmitted on the field bus, and data for transferbetween the field bus and the passive station, the process data in thepassive station is assigned to the corresponding process images in thetelegrams circulating on the field bus.
 14. Method according to claim 1,wherein the assignment table in the passive station is generated duringan initialization phase.
 15. Method according to claim 14, wherein inthe initialization phase the active station reports to the passivestation the logical addresses of data relevant for the passive stationsin the circulating telegrams and the passive station assigns to thelogical addresses respective physical addresses of corresponding datastorage areas in the passive station.
 16. Method according to claim 15,wherein the serial bus is a field bus, data in the active station isprovided in the form of process images for control tasks, data in thepassive station is provided in the form of process data, telegrams withthe process images are transmitted on the field bus, and for datatransfer between the field bus and the passive station, the process datain the passive station is assigned to the corresponding process imagesin the telegrams circulating on the field bus.
 17. Method according toclaim 14, wherein the serial bus is a field bus, data in the activestation is provided in the form of process images for control tasks,data in the passive station is provided in the form of process data,telegrams with the process images are transmitted on the field bus, andfor data transfer between the field bus and the passive station, theprocess data in the passive station is assigned to the correspondingprocess images in the telegrams circulating on the field bus.
 18. Methodaccording to claim 1, wherein the serial bus is a field bus, data in theactive station is provided in the form of process images for controltasks, data in the passive station is provided in the form of processdata, telegrams with the process images are transmitted on the fieldbus, and for data transfer between the field bus and the passivestation, the process data in the passive station is assigned to thecorresponding process images in the telegrams circulating on the fieldbus.
 19. A serial bus system comprising: at least one active station(2), wherein the active station (2) transmits data in the form ofcirculating telegrams, each telegram having a logical address and a dataportion; at least one passive station, the passive station having anassignment table containing logical addresses of circulating telegrams,the logical addresses being correlated with physical addresses of datastorage areas of the passive station; and an assignment unit, which,while a circulating telegram is passing through the passive station,compares the logical address of the circulating telegram with thelogical addresses stored in the assignment table and performs a dataexchange between the assigned data storage area and the circulatingtelegram, if there is a match, circulating telegrams flowing in acircular path formed by the serial bus, with each circulating telegramstarting at an active station, sequentially moving from the activestation to the passive station on the serial bus and, after sequentiallypassing through the passive station on the serial bus, returning to theactive station from which it originated.
 20. Bus system according toclaim 19, wherein the assignment table has register data sets, each setgiving a logical address of data transmitted in a telegram circulatingon the serial bus, a physical address of the data storage area assignedto the data in the passive station, and a data transmission type. 21.Bus system according to claim 20, wherein the serial bus is a field bus,data in the active station is provided in the form or process images forcontrol tasks, and data in the passive station provided in the form ofprocess data, the assignment unit of the passive station connects thedata storage area in the passive station, which contains the processdata, to the field bus in order to assign the process data in the datastorage area to the process images contained in the telegrams on thefield bus for a data exchange between the field bus and the passivestation.
 22. Bus system according to claim 21, wherein the activestation has a field-bus switch-on device that performs the transfer ofthe process images between a data processing unit in the active stationand the field bus.
 23. Bus system according to claim 19, wherein theserial bus is a field bus, data in the active station is provided in theform of process images for control tasks, data in the passive station isprovided in the form of process data, the assignment unit of the passivestation connects the data storage area in the passive station, whichcontains the process data, to the field bus in order to assign theprocess data in the data memory storage area to the process imagescontained in the telegrams on the field bus for a data exchange betweenthe field bus and the passive station.
 24. Bus system according to claim23, wherein the active station has a field-bus switch-on device thatperforms the transfer of the process images between a data processingunit in the active station and the field bus.
 25. A switch-on unit in apassive station in a serial bus system for performing a data transferwith a bus to which at least one active station and at least one passivestation are attached, wherein the active station transfer the data inthe form of circulating telegrams, each circulating telegram having alogical address and a data portion, each circulating telegram flowing ina circular path formed by the serial bus, with each circulating telegramstarting at an active station, sequentially moving from the activestation to the one passive station on the serial bus and, aftersequentially passing through each passive station on the serial bus,returning to the active station from which it originated, the switch-onunit comprising: an assignment unit having an assignment table, theassignment table containing logical addresses of circulating telegrams,the logical addresses being correlated with physical addresses of datastorage memory areas of the passive Station; and a control unit, whilethe circulating telegram is passing through the passive station,comparing the logical address of each circulating telegram with thelogical addresses stored in the assignment table and performing a dataexchange between the assigned data storage area and the circulatingtelegram, if there is a match.
 26. Switch-on unit according to claim 25,wherein the assignment table has register data sets, each set giving alogical address of data transmitted in a telegram circulating on theserial bus, a physical address of the data storage area in the passivestation assigned to the data, and a data transmission type. 27.Switch-on unit according to claim 26, wherein each register data setcontains a starting address of data in the data portion of the telegramcirculating on the serial bus, an ending address of the data in the dataportion or a data length in the data portion in the telegram circulatingon the serial bus, a starting address of the assigned data storage areain the passive station, and a label characterizing the data transmissiontype as a read and/or write process.
 28. Switch-on unit according toclaim 27, wherein the assignment table is generated during aninitialization phase.
 29. Switch-on unit according to claim 26, whereinthe assignment table is generated during an initialization phase. 30.Switch-on unit according to claim 25, wherein each register data setcontains a starting address of data in the data portion of the telegramcirculating on the serial bus, an ending address of the data in the dataportion or a data length in the data portion of the telegram circulatingon the serial bus, a starting address of the assigned data storage areain the passive station, and a label characterizing the data transmissiontype as a read and/or write process.
 31. Switch-on unit according toclaim 30, wherein the assignment table is generated during aninitialization phase.
 32. Switch-on unit according to claim 25, whereinthe assignment table is generated during an initialization phase.